Quake swarm not related to Cascadia

An earthquake swarm off the Del Norte County coast may make some nervous, but the cluster that occurred 127 to 141 miles west of Crescent City on Tuesday isn’t indicative of a larger temblor that would spawn a tsunami, according to Humboldt State University geology professor Lori Dengler.

The earthquake cluster also wouldn’t relieve fault stress on the Cascadia Subduction Zone, Dengler said Friday. In fact, she said, the quakes were far from the subduction zone.

“These quakes had nothing to do with the subduction zone,” Dengler said. “They’re actually related to the stretching of the Gorda ridge and not to the stuck part where the Gorda plate is going under the North American plate.”

On Tuesday, at 8:38 a.m., the Redwood Coast Tsunami Work Group notified people of the earthquake cluster via its Facebook page. According to the U.S. Geological Survey, the cluster began with a magnitude 4.3 earthquake at 5:16 a.m., followed by a 5.2 temblor 12 minutes later and a 3.4 magnitude quake at 7:33 a.m. A second 4.3 magnitude quake struck at 7:35 a.m. followed by a 4.2 at 7:42 a.m. and a 5.6 at 7:44 a.m.

“Preliminary analysis places all of them on N-S oriented normal faults within the Gorda plate, 127 to 141 miles west of Crescent City,” the Redwood Coast Tsunami Work Group stated.

At 11:08 a.m. Tuesday, a magnitude 4.9 earthquake struck in the same vicinity, the Redwood Coast Tsunami Work Group posted on Facebook.

In a Eureka Times-Standard column published Wednesday, Dengler said there were a total of 11 earthquakes ranging in magnitude from 2.8 to 5.6.

Later on Tuesday, at about 10:06 p.m., a magnitude 4.5 earthquake occurred 42 miles west of Petrolia offshore of Cape Mendocino in Humboldt County. This quake occurred on the Mendocino Fault and is also not related to the subduction zone, Dengler said.

Even if these earthquakes were on the Cascadia Subduction Zone, magnitude 4 and 5 earthquakes don’t release much energy, Dengler said.

“To release the amount of energy in a single magnitude 9 earthquake you need to have five magnitude 5 earthquakes every day for about 500 years,” she said. “Let’s just say they’re not going to do the job.”

Even though the quakes didn’t cause any damage and weren’t widely felt, they still serve as a warning that the next could be larger or closer to the coast and preparation is important, Dengler said.